feat(javascript): link assignment right-hand sides to their writes (#6206)

* feat(javascript): link assignment right-hand sides to their writes

A declaration carries its value forward, but a later assignment to a
variable, a property or through a compound operator stopped the forward
trace at the right-hand-side read. The semantic IR now records one
syntactic assignment flow per simple assignment target (ranges, operator,
owning callable), and the graph projection connects each retained read
of the right-hand side to the write occurrence through a defines
relation. Compound assignments also consume the previous value of the
target; the written value itself stays unknown, keeping the boundary of
#4342 intact.

Fixes #6193

* fix(javascript): preserve computed key assignment ownership

* test(javascript): retain nested callback assignment ownership

* docs(javascript): record assignment influence verification

* test(mcp): isolate mocks and acknowledge cancellation boundaries

* test(javascript): retain CLI diagnostics and current-main verification

---------

Co-authored-by: gensokyo-dev <342580529+gensokyo-dev@users.noreply.github.com>

src/javascript/analysis/JavaScriptAnalysisWorkerSchemas.ts

@@ -629,6 +629,7 @@ const callable = z.strictObject({
   containerScopeId: z.string(),
   bodyScopeId: z.string().nullable(),
   location: range,
+  computedKeyLocation: range.nullable(),
   returnSites: z.array(
     z.strictObject({
       returnSiteId: z.string(),